Also, hubby's computer is an even older Dell. No pci express slot.
All right if there is a momentary input then nothing that means either the mb is grounded, a bad mb, or bad psu. You eliminated the psu so we are down to mb.
If you are sure all the standoffs were placed properly that would eliminate the grounding issue.
Based on nothing staying on ( I assume that means case fans and cpu cooler fan) then that is a bad mb. Return it. for another one.